Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
A typical cellular wireless network includes a number of base stations each radiating to define a respective coverage area in which wireless communication devices (WCDs) (also known as user equipment devices (UEs), whether or not operated by an end-user), such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. (In practice, a given cell site may include an antenna structure and associated equipment that is arranged as multiple such base stations, each radiating to define a respective coverage area.) In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a WCD within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs served by the base station.
In general, a cellular wireless network may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to WCDs defining a downlink or forward link and communications from the WCDs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE)), Code Division Multiple Access (CDMA) (e.g., 1xRTT and 1xEV-DO), Wireless Interoperability for Microwave Access (WiMAX), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of WCDs, initiation of communications, handoff between coverage areas, and other functions related to air interface communication.
In accordance with the air interface protocol, each base station (and thus each coverage area) may operate on at least one coverage frequency for carrying communications wirelessly between the base station and WCDs. In particular, the air interface between the base station and WCDs may carry communications on one or more particular frequency channels, each defining one or more ranges of radio frequency spectrum within a particular frequency band (or “band class”). As such, the coverage frequency of a base station may be considered to be the one or more frequency channels on which the base station operates and/or the one or more frequency bands in which the base station operates.
The industry defines various frequency bands (or “band classes” (BCs)), some of which may be designated for frequency division multiplex (FDD) use in which uplink and downlink communications occur in different parts of the band, and others of which may be designated for time division duplex (TDD) use in which the uplink and downlink communications share the same frequency over time. Further, in accordance with industry regulations or engineering design, the frequency channels defined in these bands may vary in bandwidth based on the bands in which they are defined and may thus support different speeds of data communication.
By way of example, for LTE service, the industry currently defines BC-25 (at or around 1900 MHz) and BC-26 (at or around 800 MHz) for FDD communication, and the frequency channels provided in these bands may span 5 MHz in each direction, thus supporting up to about 25 Mbps on the downlink. In addition, the industry currently defines BC-41 for TDD communication, and the frequency channels provided in BC-41 may be 20 MHz wide, thus supporting up to about 60 Mbps on the downlink.
Also in accordance with the air interface protocol, the coverage area provided by each base station may define various logical or physical channels (e.g., through frequency division multiplexing, time division multiplexing, orthogonal frequency division multiplexing, and/or code division multiplexing) for carrying certain types of communications between the base station and WCDs. By way of example, on the downlink, the coverage area may define a reference channel for carrying a reference signal that WCDs can monitor as a way to detect and evaluate coverage of the base station, one or more control channels for carrying various system information and control messages to WCDs, and one or more traffic channels such as a shared downlink channel for carrying bearer communication traffic and other data to WCDs. And on the uplink, the coverage area may similarly define one or more control channels for carrying control messages to the base station, and one or traffic channels such as a shared channel for carrying bearer communication traffic and other data to the base station.
Further, a base station and/or associated network infrastructure may maintain a “neighbor list” that indicates other nearby base stations to which WCDs being served by the base station could potentially hand over. Such a neighbor list could specify various information about each such handover neighbor, including for instance as one or more coverage frequencies on which the neighbor operates, a network address of the neighbor, and a geographic location of the neighbor. In practice, such neighbor data could be updated from time to time as changes are made in the configuration of the network, such as when base stations are added, removed, moved, or reconfigured. For instance, the neighbor data could be updated manually by engineering input as changes are made to the network and/or through an automated process as WCDs report various detected neighbors.
In practice, when a WCD is being served by a base station on a particular coverage frequency, the WCD may monitor the reference signal from the base station and may evaluate the signal strength (e.g., signal-to-noise ratio) of that reference signal as a basis to determine when the WCD should begin considering the possibility of handover to another base station, perhaps on another coverage frequency. For instance, the WCD may determine when the reference signal strength from its serving base station falls below a predefined threshold level defined as a start-scanning threshold, at which point the WCD may start to scan for coverage on other coverage frequencies. Further, upon finding such other coverage, the WCD may then also determine when the reference signal strength from its serving base station is below another predefined threshold level (e.g., equal to or lower than the start-scanning threshold) defining a handover threshold at which the WCD may then request handover to a sufficiently strong other base station, at which point the WCD may then engage in signaling with its serving base station to request handover to the other base station.